Silica bricks are used as a refractory in building and repairing industrial furnaces, including coke ovens, hot blast stoves and glass furnaces. U.S. Pat. Nos. 4,866, 5,310,708, and 5,496,780, which are incorporated by reference herein in their entirety, disclose the use and properties of silica refractory bricks.
There has been very little recent improvement in the manufacture of the conventional silica refractory manufactured for use in glass furnaces. Conventional silica bricks are typically manufactured by mixing quartz grains with calcium oxide binder, pressing the mixture into bricks and firing the bricks to temperatures up to 1700.degree. C. to allow more than about 95% of the quartz to transform to cristobalite and tridymite. This transformation avoids the problems associated with expansion changes of alpha and beta quartz and allows for faster heating of the furnaces in which the brick is used. Conventional silica refractory bricks used in glass furnaces contain about 2.5% to 3% calcium which acts as a binder between the silica grains.
Environmental considerations, particularly the desire to lower the emission of nitrogen oxides from industrial furnaces, have driven the replacement of air-fuel firing of glass furnaces with oxygen-fuel firing. The change from air-fuel firing to oxygen-fuel firing has increased alkalis in the furnace environment, for example, sodium hydroxide in glass melting furnaces. Consequently, the increased alkalis in the furnace environment have caused additional corrosion of fused silica refractories. In some instances, the switch from air-fuel firing to oxygen-fuel firing increases the alkalis such as sodium hydroxide by a factor of four in the glass furnace.
Thus, alkalis in the furnace have been identified as being detrimental to silica refractory bricks. The crowns of glass furnaces are typically made of silica refractory bricks. The surface of an oxygen-fuel fired glass furnace crown made from silica refractory bricks can be degraded by about one to two inches per year. The typical life of a crown of a float glass furnace using oxygen-fuel firing is about seven years, which is a reduction of about five years compared with a furnace using air fuel-firing.
Alternatives to conventional silica refractories include alumina-zirconia-silica (AZS) refractories and amorphous silica refractories that do not contain any binder. However, one disadvantage with these alternative refractories is that they are much more expensive than the conventional silica refractory bricks made from quartz grains and contain calcium oxide binder.
There is a distinct need for a silica refractory that is resistant to alkali attack and degradation in oxygen-fuel fired furnaces, but is less expensive than AZS and amorphous silica refractory materials. It would also be advantageous to provide a refractory brick that could be utilized for long spans of glass furnace crowns and would require very little maintenance.